1. Field
The present invention relates generally to communication, and more specifically to techniques for adjusting a signal quality (SIR) target used for power control in wireless communication systems.
2. Background
In a Code Division Multiple Access (CDMA) system, the total transmit power available for a base station is typically indicative of the total downlink capacity for that base station. This is because the base station may concurrently transmit data to multiple terminals on the same operating band at any given moment. On the downlink, a portion of the total transmit power may be allocated to each active terminal such that the aggregate transmit power used by the base station for all active terminals is less than or equal to the total transmit power.
To maximize downlink capacity, a power control mechanism is typically used to minimize power consumption and interference while maintaining a specified level of performance. This power control mechanism is typically implemented with two power control loops—an “inner” power control loop (or simply, the inner loop) and an “outer” power control loop (or simply, the outer loop). For a data transmission to a terminal, the inner loop adjusts the transmit power for the data transmission such that the received SIR for this data transmission at the terminal is maintained at a particular signal quality target, which may be given as a signal-to-interference ratio (SIR) target, an energy-per-bit-to-total-noise ratio (Eb/Nt) target, or some other measurement. SIR target is commonly used in W-CDMA, and Eb/Nt target is commonly used in IS-95 and IS-2000. For simplicity, the term “SIR target” generically refers to signal quality target in the following description. This SIR target is also often referred to as the power control setpoint. The outer loop adjusts the SIR target so as to maintain the specified level of performance, which may be quantified by a particular target block error rate (BLER), frame error rate (FER), packet error rate (PER), bit error rate (BER), or some other measure. BLER is commonly used in W-CDMA, and FER is commonly used in IS-95 and IS-2000. For simplicity, the term “BLER” generically refers to level of performance or quality of service (QoS) in the following description.
At the start of a communication session, the SIR target is normally set to an initial value that is often selected to be high enough to ensure reliable communication even for the worst-case scenario (i.e., the worst communication channel condition). The SIR target is then adjusted up or down as each block of data is received and decoded by the terminal. Conventionally, the SIR target is decreased by a small down step (ΔDN) if a data block is decoded correctly and increased by a large up step (ΔUP) if the data block is decoded in error. The ratio between the up step and down step is determined by the target BLER (e.g., ΔUP=99·ΔDN for a target BLER of 1%). Since the initial value for the SIR target is typically high (to ensure reliable communication) and since the down step is small, it typically takes the outer loop a prolonged period of time to adjust the SIR target to the final value needed to achieve the target BLER. During this period of time prior to convergence of the outer loop, excessive transmit power is used and downlink capacity is wasted. This problem is exacerbated if (1) the target BLER is a small value and the down step is correspondingly small, (2) data is transmitted intermittently instead of continuously, and/or (3) data is transmitted in bursts and the SIR target needs to be re-initialized for each burst.
There is therefore a need in the art for techniques to reduce the amount of time needed to adjust the SIR target to the final value.